Stent

ABSTRACT

A stent and a method of making it from a wire, which method includes winding the wire on a mandrel, heating to form a coiled spring, and reversing the winding direction of the coiled spring to form the reversed coiled spring stent. The stent so formed may be reheated over a special mandrel so as to partly relax the outer portion of some or all of the stent coils. The stent may be made up of two or more sections, with adjoining section wound in opposite senses. Such a stent may be deployed by winding the stent onto a catheter, immobilizing the two ends of the wire and one or more intermediate points, bringing the stent to the location where it is to be deployed, and releasing first the intermediate point or points and then the end points. The release of the wire may be accomplished by heating the thread immobilizing the wire so that the thread breaks and releases the wire.

RELATED PATENT APPLICATIONS

This application is a continuation of prior application No. 09/519,163filed Mar. 6, 2000, now U.S. Pat. No. 6,666,881, which is a continuationof Ser. No. 09/361,704 filed Jul. 27, 1999, now abandoned, which is acontinuation of Ser. No. 08/679,606 filed Jul. 11, 1996, now abandoned,which is a continuation of Ser. No. 08/297,275, filed Aug. 26, 1994(abandoned), which is a continuation of Ser. No. 08/029,493 filed Mar.11, 1993 (abandoned).

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to stents and, more particularly, tomethods of fabricating and deploying stents.

The term “stent” has come into widespread use to denote any of a largevariety of spring-like support structures, in the form of a tube whichis open at both ends, which can be implanted inside a blood vessel orother tubular body conduit, to help keep the vessel or conduit open.Stents may be used following balloon angioplasty to prevent restenosisand may, more generally, be used in repairing any of a number of tubularbody conduits, such as those in the vascular, biliary, genitourinary,gastrointestinal and respiratory systems, among others, which havenarrowed, weakened, distorted, distended or otherwise deformed,typically as a result of any of a number of pathological conditions.

An effective stent must possess a number of important and very specificcharacteristics. Specifically, the stent should be chemically andbiologically inert to its surroundings and should not react with, orotherwise stimulate, the living tissues around it. The stent mustfurther be such that it will stay in the correct position and continueto support the tubular body conduit into which it is implanted overextended periods of time. Further, the stent must have the ability toreturn to its prescribed in-place diameter after the stent diameter hasbeen significantly reduced prior to its insertion, usually tightlywrapped on a catheter, into the tubular body conduit.

These requirements limit the suitable metal stent materials to just afew metals and alloys. To date, it has been found that various alloys ofnickel and titanium (hereinafter “nitinol”), with or without certaincoatings, have the desired properties and are considered suitable foruse in stent applications.

Specifically, nitinols, with or without special coatings, have beenfound to be chemically and biologically inert and to inhibit thrombusformation. Nitinols are, under certain conditions, also superelasticwhich allows them to withstand extensive deformation and still resumetheir original shape. Furthermore, nitinols possess shape memory, i.e.,the metal “remembers” a specific shape fixed during a particular heattreatment and can resort to that shape under proper conditions.Shape-memory alloys can be formed into a predetermined shape at asuitable heat treatment temperature. At temperatures below thetransition temperature range (“TTR”) certain nitinol alloys are in theirmartensite phase wherein they are highly ductile and may be plasticallydeformed into any of a number of other shapes. The alloy returns to itsaustenite phase, returning to its original predetermined shape uponreheating to a temperature above the transition temperature range. Thetransition temperature varies with each specific combination ratio ofthe components in the alloy.

The superelasticity of nitinols and their shape memory characteristicsmakes it possible to fabricate a stent having the desired shape anddimensions. Once formed, the stent can be temporarily deformed into amuch narrower shape for insertion into the body. Once in place, thestent can be made to resume its desired shape and dimensions. Certainalloys of nickel and titanium can be made which are plastic attemperatures below about 30° C. and are elastic at body temperaturesabove 35°C. Such allovs are widely used for the production of stents formedical use since these nitinols are able to resume their desired shapeat normal body temperature without the need to artificially heat thestent.

While such stents have been proven effective, they continue to sufferfrom a number of disadvantages. First, there is, in certain cases, atendency for tissue to grow in the gads between adjoining loops of thestent. Over time, such growth could lead to the constriction, or eventhe complete closure, of the tubular body conduit in which the stent wasintroduced in order to keep open. A continuous, gap-free, tube structurewith no gaps would eliminate such undesired tissue growth. However, arigid tube would lack the highly desirable flexibility which a coiledspring configuration offers.

Another disadvantage is that the techniques for locating stents in abody conduit are such that the stents are often installed at a locationwhich is not precisely the intended optimal location.

There is thus a widely recognized need for, and it would be highlyadvantageous to have, a stent which would be suitably flexible but whichwould significantly reduce, or even eliminate, the possibility ofundesired tissue growth between the coils of the stent.

There is further a widely recognized need for, and it would also behighly advantageous to have, a technique for installing stents whichwould allow the stent to be located at precisely the desired location,either by controlling the stent design or by devising adequate methodsfor its accurate release. Furthermore, in those cases where the “shapememory” characteristic is used and the stent is to be heated in itsfinal location in the body to cause it to resume its memorized shape, itis desired and advantageous to have a way of heating the stent whichsignificantly reduces, or even eliminates, the chance of damagingsurrounding tissue through heating which is conducted for too longand/or at temperatures which are too high.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method offabricating a stent from a wire, comprising: (a) winding the wire on afirst mandrel; (b) heating the wound wire to form a coiled spring; and(c) after the coiled spring has cooled sufficiently, reversing thewinding direction of the coiled spring to form the stent.

Further according to the present invention there is provided a stentcomprising a coiled wire characterized in that the wire includes atleast one section which is wound in one sense and at least one sectionwhich is wound in the opposite sense, deployment of said stent takingplace by tightly winding the stent onto a catheter and subsequentlyallowing the stent to resume its normal dimensions.

Still further according to the present invention there is provided amethod of deploying a stent in a desired location, comprising: (a)tightly winding the stent onto a catheter; (b) immobilizing at least twotie-down points on the stent using a disconnectable thread; (c) bringingthe stent to the desired location where the stent is to be deployed; (d)causing the thread to disconnect at one or more of the tie-down points,thereby releasing the tie-down point, wherein said disconnectable threadis meltable and said thread is disconnected by heating the thread so asto cause the thread to melt.

Further yet according to the present invention there is provided amethod of heating a nitinol stent to cause the stent to shift from itsmartensite phase to its austenite phase and to monitor the phase change,comprising: (a) electrically connecting the stent to an electrical powersupply; (b) supplying electrical current to the stent; (c) sensing achange in at least one electrical property to indicate the phase change;(d) controlling the current in response to the change.

The present invention successfully addresses the shortcomings of thepresently known stents and their methods of deployment by providing astent which is suitably flexible but which is sufficiently tight so asto eliminate the gaps between adjoining windings of the stent, therebysignificantly reducing or even eliminating the possibility ofundesirable growth of tissue between winding of the stent. The presentinvention further offers stents and associated deployment techniqueswhich make it possible to accurately install the stent in a specificlocation of a body tubular conduit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a perspective view of a single winding of a prior art stent;

FIG. 2 is a perspective view of a single winding of stent according tothe present invention which was obtained by reversing the winding of astent such as that in FIG. 1;

FIG. 3 is a side cross sectional view of a stent undergoing reheatingaccording to the present invention, on a mandrel having two sections,each with a different heat sink capacity;

FIG. 4 is a perspective view of a stent wound and immobilized on acatheter, according to the prior art;

FIG. 5 is a close-up side cross sectional view of a portion of thesystem of FIG. 4;

FIG. 6 is a perspective view of one embodiment of a stent according tothe present invention showing two oppositely wound sections;

FIG. 7 is a schematic side view of the stent of FIG. 6 with reference toa catheter on which the stent is delivered to its desired location afterrelease of the intermediate point;

FIG. 8 is a schematic side view of the stent of FIG. 6 when woundtightly on a catheter on which the stent is delivered to its desiredlocation;

FIG. 8A is a side view of a catheter such as might be used in FIG. 8;

FIG. 8B is a side view of the catheter of FIG. 8A with the stent woundon the catheter;

FIG. 8C is a side view of the expanded stent after its release;

FIG. 9 is a perspective view of another embodiment of a stent accordingto the present invention showing a plurality of oppositely woundsections;

FIG. 10 is a schematic side view of the stent of FIG. 9 with referenceto a catheter on which the stent is delivered to its desired locationwith the stent partly released;

FIG. 11 is a schematic side view of the stent of FIG. 9 when woundtightly on a catheter on which the stent is delivered to its desiredlocation;

FIG. 12 is a perspective view of the embodiment of FIGS. 9–11 showingone method of immobilizing the stent;

FIG. 13 is a close-up perspective cross sectional view of one portion ofthe system of FIG. 12 showing a tie-down of an intermediate point;

FIG. 14 is a close-up perspective cross sectional view of one portion ofthe system of FIG. 12 showing a tie-down of an end point;

FIG. 15 is a perspective view of a variation of the embodiment of FIG.9, showing a stent wherein the immobilization is effected in somewhatdifferent fashion;

FIG. 15A is a side view of a catheter such as might be used in FIG. 15;

FIG. 15B is a side view of the catheter of FIG. 15A with the stent woundon the catheter;

FIG. 15C is a side view of the expanded stent as it would appear afterit has been released from the catheter;

FIG. 16 is a laid-flat view of an embodiment according to the presentinvention wherein the stent coils are encased by a film of flexiblematerial;

FIG. 16A is a view of another embodiment of the device of FIG. 16,including an integral immobilization thread;

FIG. 16B is a side view of the device of FIG. 16A as it would appearwhen wound onto a catheter;

FIG. 17 is an end cross sectional view of the stent of FIG. 16 whentightly wound onto a catheter;

FIG. 18 is a side cross sectional view of the stent of FIG. 16 whentightly wound onto a catheter;

FIG. 19 is a side cross sectional view of another embodiment of a stentaccording to the present invention when tightly wound about a catheter(not shown);

FIG. 20 is a side cross sectional view of the embodiment of FIG. 19 whenunwound for deployment;

FIG. 21 is a side view of stent featuring neck-down regions and twosections connected by a coil of low pitch;

FIG. 22 is an end cross-sectional view of the stent of FIG. 21;

FIG. 23 is a schematic cross sectional side view of a catheter showingone embodiment of a technique for releasing the stent (not shown) usinga single electrical circuit;

FIG. 24 is as in FIG. 21 except that two electrical circuits are used toprovide for the sequential release of various points of the stent;

FIG. 25 is a circuit diagram for the electrical heating and phase shiftsensing of a stent according to the present invention, for the automaticdisconnection of the heating circuit upon, or at an appropriate timefollowing, the detection of the phase shift.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is of improved stents and of methods of making anddeploying them which can be used to increase the effectiveness ofstents.

The principles and operation of stents and related methods according tothe present invention may be better understood with reference to thedrawings and the accompanying description.

Referring now to the drawing, FIG. 1 illustrates a single winding of aconventional stent. In many applications, it is important to preciselycontrol the flexibility of the stent as well as the interloop spacingand tightness. A number of factors must be considered in selecting theproper flexibility and interloop spacing. First, the stent must besufficiently flexible to follow the natural shape and dimensions of thebody conduit into which it is installed without undue stress. The stentmust also be sufficiently flexible to adequately follow the variousmovements of the conduit. These requirements tend to indicate that acoiled, or spring-like, structure be used.

However, the stent must not be too loose since this may erode its bodyconduit support function and since when the stent loosens significantinterloop gaps are formed which tend to encourage the growth ofsurrounding tissue into the separations. Such ingrowth may have seriousadverse consequences.

A stent is typically made by first tightly winding a wire of a suitablematerial, such as nitinol, on a mandrel. The assembly is then heated toa suitable temperature so as to impart to the stent its desired shape.However, during the heating process, the mandrel is also heated, whichbrings about its expansion and leads to the formation of a stent withloops which are somewhat separated from one another. Such separationsare undesirable in certain applications.

These interloop gaps can be eliminated and the stent can be stiffenedsomewhat by reversing the winding direction of the stent after it hascooled sufficiently. Shown in FIG. 2 is the single stent winding of FIG.1 after it has been reversed. Thus, what, prior to reversal (FIG. 1),was the left end of the loop, 10, is, after reversal (FIG. 2), the rightend of the loop, 10, while what, prior to reversal (FIG. 1), was theright end of the loop, 12, is, after reversal (FIG. 2), the left end ofthe loop, 12. As will be appreciated, the reversal puts each loop inelastic deformation and thereby causes adjoining loops to press togetherand eliminates interloop gaps.

Under certain conditions a stent made by reversing the winding directionas described above may be overly rigid for a specific application. Insuch a case, the rigidity of the stent may be reduced to any desiredlevel by following a reheating procedure described below.

The reversed stent 20, whose rigidity is to be reduced, is mounted ontoa mandrel 22 (FIG. 3) which may or may not be the mandrel previouslyused to give the stent its original shape. Stent 20 and mandrel 22 arereheated at a suitable temperature above the transition point but thereheating is allowed to continue only long enough to allow the outsideportion of the stent (indicated in FIG. 3 as the unhatched portion) toapproach the reheating temperature and therefore to relax, while theportion of the stent near the relatively cool mandrel (indicated in FIG.3 by hatch marks) stays at significantly lower temperatures, does notrelax, and continues to have its original rigidity. In this way thereheated stent, upon cooling, displays a flexibility which isintermediate between those of the unreheated stent and a stent which iscompletely relaxed, but without opening up gaps between the stent loops.

The duration of the reheating must be carefully controlled to achievethe proper degree of relaxation. The reheating time will be influencedto a large degree by the heat properties of the mandrel. A mandrel whichhas high heat sink capacity, such as the left-hand portion of themandrel of FIG. 3, can absorb considerable heat and keep the stent atlow temperatures for a relatively long time.

By contrast, a mandrel which has low heat sink capacity, such as theright-hand portion of the mandrel of FIG. 3, can absorb very little heatand will not keep the stent at low temperatures but rather will allowthat portion of the stent overlying it to quickly reach the overallheating temperature and undergo complete relaxation.

Advantage may be taken of this property to reheat different portions ofa stent to different extents so as to achieve a final product which hasa certain rigidity in one or more sections and is relaxed and featuressignificant interloop gaps in other sections. Typically, it may beuseful to have significant interloop gaps between the windings near eachend of the stent to facilitate the suturing of the stent in place.

It will be appreciated that a stent having regions of differingrelaxation characteristics can also be achieved by heating the differentsegments to different temperatures and times, such as by use of asegmented heater or furnace.

Conventional stents, as well as the reversed stents according to thepresent invention described above, must be accurately placed in aspecific location in the body conduit in order to be most effective. Acommon placement technique currently used is illustrated in FIGS. 4 and5. Stent 20 is tightly wound around a catheter 24, which typicallyfeatures helical grooves 26 sized and shaped to accommodate stent 20 inits tightly wound configuration.

The two ends of stent 20 are typically bulbed, i.e., the ends feature aslightly enlarged diameter. Each end of stent 20 is immobilized by athread 28 which is anchored by wrapping around catheter 24 severaltimes. Thread 28 is wrapped over the end of stent 20 as shown in FIG. 5.Catheter 24 features a small diameter bore 30 through which runs arelease wire 32. Portions of thread 28 enter transversely into bore 30near the bulbed end of stent 20 and thread is connected with releasewire 32 (see FIG. 5) so that as long as release wire 32 is in placethread 28 immobilizes the end of stent 20. When both ends of stent 20are so immobilized, stent 20 is effectively prevented from unwinding andresuming its preset shape.

To deploy stent 20 in the body, catheter 24 is first brought to theappropriate position. Release wire 32 is then pulled, thereby releasingthe ends of stent 20. Stent 20 then immediately proceeds to unwind,enlarge and install itself into the body tubular conduit while gettingshorter in proportion to the diameter growth, as is the case for a stenthaving adjoining loops which contact each other. However, in the processof unwinding, stent 20 assumes a final position which is somewhatarbitrary, within its original length, and which depends, to someextent, on the local resistance encountered to the unwinding in theuneven blood vessel. The lack of certainty in the accurate placement ofthe stent often degrades its effectiveness. Hence, it is quite importantto be able to release the stent with a high degree of accuracy.

Furthermore, the unwinding action of a stent of conventional design isaccompanied by the rapid turning through many cycles of the stent coils.Such a turning could have a detrimental effect on surrounding tissuesince the rapid and prolonged turning could abrade or otherwise damagethe interior walls of body vessels in which the stent is released.

Accordingly, a stent according to one embodiment of the presentinvention is made up of a coiled wire which is characterized in that thewire includes at least one section which is wound in one sense and atleast one section which is wound in the opposite sense. Preferably, thestent includes two sections with each of these sections representingsubstantially one half of the stent. An example of such a stent is shownin FIGS. 6–8.

Stent 120 has a central point 40 where the winding direction changes,and two end points 42. To place stent 120 in a body conduit, stent 120is first tightly wound onto catheter 24 and end points 42 areimmobilized using release wire 32 as described above in the context ofFIGS. 4 and 5, or in any other suitable manner. In addition, centralpoint 40 is also immobilized in a similar manner, but using a secondrelease wire 33.

To place stent 120, catheter 24 is first brought to the proper location.Next, central point 40 is released by using second release wire 33. Thisallows stent 120 to unwind without any axial displacement, since the twoends 42 are still immobilized. As stent 120 unwinds it assumes its fulldiameter and is firmly installed onto the inner walls of the bodytubular conduit.

At this point the two end points 42 are released by using release wire32, freeing stent 120 from catheter 24, and allowing the latter to bewithdrawn. Since stent 120 is already fully unwound and firmly implantedin the body conduit prior to the release of end points 42, stent 120does not move upon the release of end points 42 and remains firmlyinstalled in the correct position. Shown in FIGS. 8A, 8B and 8C are moredetailed views of catheter 24 and stent 120 as they might appear in anactual application.

In another embodiment of stents according to the present inventionshowing in FIGS. 9–12, stent 220 is made up of several sections withadjoining sections wound in opposite directions. Preferably, adjoiningloops of stent 220 are wound in opposite directions, with intermediatepoints 140 representing the regions where winding directions change. Tomake such a stent, a catheter can be used which features a series ofsuitably placed pins or protrusions. The wire is wound about the mandreland use is made of the pins or protrusions to wrap the wire around thesein order to reverse the winding direction.

Shown in FIG. 12 is one scheme for attaching stent 220 to catheter 24.Here a first release wire 132 immobilizes the two end points 42 andapproximately one half of intermediate points 140, while a secondrelease wire 133 serves to immobilize the balance of intermediate points140. Each of release wires 132 and 133 is preferably located in its ownbore, 232 and 233, respectively. The release of such a stent is notaccompanied by the rapid and prolonged turning of the coils of the stentbut is, rather, achieved by minimum and uniform turning of the coilsthrough approximately two turns before the stent is fully deployed inthe body vessel.

FIGS. 13 and 14 depict possibilities for the actual immobilization of anintermediate point 140 and an end point 42, respectively.

Another embodiment of a stent according to the present invention isshown in FIG. 15, where at the end points and in the vicinity of eachwinding direction change, stent 320 features a kink or depression 50 inthe otherwise circular cross section of the stent. The kink ordepression 50 allows stent 320 to be immobilized on a catheter (notshown) by inserting a release wire (not shown) above kink or depression50 (see FIG. 15).

As can be better seen in FIGS. 15A and 15B, catheter 24 preferablyfeatures slots 25 which accommodate the kinked portions of stent 320 sothat release wires 32 and 33 can pass on the outside of the kinkedportions and serve to immobilizes stent 320. FIG. 15C shows stent 320 asit would appear after release from catheter 24.

Other variations and improvements of methods of immobilizing andreleasing stents, whether conventional, or those according to thepresent invention, may be envisioned.

When a stent is to be inserted deep into the body, the catheter used indeploying the stent is necessarily very long and may need to follow ahighly convoluted path on its way to the desired deployment location. Ifthe stent is to be released from the catheter by pulling on the releasewire, the friction between the release wire and its bore may besufficiently high that pulling the release wire will result in thedeformation of the entire catheter and bring about the displacement ofthe catheter tip on which the stent is wound. This, in turn, couldresult in the improper placement of the stent.

One way of avoiding this difficulty is demonstrated in FIGS. 23 and 24.Here the release wire is an electrically conducting wire which, unlikethe release wires described above, is not movable but is, rather, usedto conduct a small electric current upon activation by the operator. InFIG. 23, a pair of threads 28 are shown, each of which is used toimmobilize a certain point on the stent (not shown). Thread 28 is incontact with a heat producing element 60 which forms a part of theelectrical circuit. Heat producing element 60 may be a resistor whichconverts electrical energy into heat. Thread 28 is made of a materialsuch that when heat producing element 60 is activated, thread 28 iscaused to melt thereby releasing the stent.

In the embodiment of FIG. 24 catheter 24 features two circuits, ratherthan one. This makes it possible to sequentially release various pointsof the stent, for example, as described above. As will readily beappreciated, the basic concept can be used in a variety of related ways.For example, thread 28 can be caused to break or disconnect by cutting,by chemical reaction, and the like.

Nitinols of certain composition have transition temperatures rangeswhich are such that the nitinol is in its martensite phase, and isplastic, at temperatures of about 30° C. and is in its austenite phase,and highly elastic, at or above body temperatures, above about 37° C.Such alloys are useful since stents made from them can be tightly woundabout a catheter at room temperature and can then automatically resumetheir desired shape at normal body temperature without the need toartificially heat the stent.

However, this technique suffers from a disadvantage in that the stentmay heat to body temperature prematurely, that is, before it is placedin its intended position, and may thus suffer undesirable stresses andpermanent deformation. It is, thus, useful to employ nitinols which havea transition temperature range above body temperature (about 37° C.)such that the stent must be heated to a temperature above bodytemperature in order to convert the nitinol into its austenite phase.

In such cases conventional techniques call for the heating of stentthrough the circulation of hot liquids through the catheter used toplace the stent. A difficulty with such techniques is that a liquid mustbe injected having a temperature which is sufficiently high so as toreach the stent at a temperature which is sufficiently high to raise thestent temperature above the required TTR. Especially where a longcatheter must be used to reach remote body vessels, the injected liquidtemperature may be high enough to cause damage to blood and other bodytissues.

The problem is compounded by uncertainty as to when the heating shouldbe discontinued, since it is difficult to know precisely when thenitinol reaches the desired temperature. As a result, there is atendency to overheat the stent, which further incurs the risk ofoverheating and damaging body tissues.

To overcome these shortcomings, it is proposed that the stent be heatedelectrically and that advantage be taken of the differences in theproperties of nitinols in their martensite and austenite phases to sensethe change of phase of the nitinol to automatically regulate theheating.

Depicted in FIG. 25 is a circuit diagram of a stent heating andmonitoring system according to the present invention. The principles andoperation of such a system may be better understood with respect to aspecific example described next. It is to be understood that the exampleis illustrative only and does not, in any way, limit the scope of theinvention.

It is known that both the resistivity and the thermal conductivity of anitinol alloy in its austenite phase are different than in itsmartensite phase. For example, for a particular nitinol, theresistivities are 70 and 100 μohm·cm for the martensite and austenitephases, respectively. The thermal conductivities for the same nitinolare 0.085 and 0.18 Watt/cm·C° for the martensite and austenite phases,respectively

In a system according to the present invention, stent 100 would beelectrically connected to a power source 102, such as a 12V battery. Anappropriate first resistance 104, for example 0.018 ohm, and a secondresistance 108, for example 0.036 ohm, are provided to put a desirablevoltage drop in the martensite phase, say 7.5V, across stent 100, havingresistance of 0.09 ohm (0.5 mm diameter and 100 mm length).

When stent 100 shifts into its austenite phase its resistance willincrease to 0.13 ohm and the voltage drop across stent 100 will increaseto 9V. The sharp change in voltage is an excellent indication that stent100 has shifted into its austenite phase and can be used to control theend of the heating process, either cutting off heating immediately upondetecting the voltage change or at a certain fixed or calculated timethereafter.

For example, as shown in FIG. 25, the circuit can further include atransistor gate 106 with a threshold of 2.5V. As long as stent 100 is inits martensite phase the potential on transistor gate 106 will be 3Vwhich is above the threshold so that the circuit will be closed. As soonas the austenite phase is reached the potential on transistor gate 106drops to 2V, below its threshold, causing the circuit to open and theheating to be discontinued. The circuit may further have means (notshown) to continue heating beyond this point for a suitable time and ata suitable rate. It should be appreciated that a similar system can beused wherein the current drawn, rather than the voltage drop, is sensedand used to indicate the phase transition.

In some cases it is desirable that the stent form a continuous wall.This may be accomplished by encasing the wire making up the stent in athin plastic envelop 70 (FIG. 16) which will provide the continuous wallwhen the stent is in position. Shown in FIGS. 17 and 18 are an end viewand a side view, respectively, of stent 20 enveloped in plastic envelop70, as it would appear when stent 20 is tightly wound on catheter 24.

Another embodiment of an encased stent is shown in FIGS. 16A and 16B.The stent is as shown in FIG. 16 with the addition of a special releaseloops 21, preferably made of a suitable plastic material and areconnected to plastic envelope 70 in some suitable fashion, which can beused (see FIG. 16B) to engage release wire 32 and immobilize theintermediate points of stent 20. The ends of stent 20 can be immobilizedas described above.

Yet another embodiment of an encased stent for effecting a continuouswall upon deployment is shown in FIGS. 19 and 20. In this embodiment ametal core 80, preferably made of nitinol, is encased in a shapedenvelope 82, preferably of a suitable plastic, which allows the stent tobe tightly wound on the catheter and which forms a continuous surfacewhen the stent is unwound. Unlike the configurations of FIGS. 16–18, inthe configuration of FIGS. 19 and 20, the envelope is not continuous anddoes not directly connect adjoining coils. Rather, the wire making upthe stent is enveloped in a suitable material, such as plastic, whichfeatures an extension such that, when deployed, the extension serves tobridge the gap between adjoining coils of the stent.

The configuration shown in FIGS. 19 and 20 is such that when the stentexpands and its metal core loops are separated from each other (FIG. 20)the stent retains its continuous sealed wall. Thus, a stent is obtainedwhich features continuous walls and which is substantially the samelength when wound onto the catheter for delivery and placement as whenfully deployed in the body vessel. It should be noted that such aconfiguration may be useful even without reversing of the windingdirection, since a sealed wall is maintained even when adjoining loopsare not completely contiguous.

It is to be noted that a stent according to the present invention,especially one featuring a continuous wall supported on a metal coilframe, as described above, is highly desirable in that such a structureis able to support the body vessel and prevent tissue ingrowth withoutundue interference with the normal flow of blood and other bodilyfluids. The latter characteristic is achieved through use of very thincoils and thin connecting walls enveloping the stent coils.

In addition, the profile and configuration of the stent can be adjustedso as to further minimize the flow friction of fluids flowing inside thestent and reduce turbulence. For example, the distal ends of the stentcan be made to have large coil diameters than the rest of the coils sothat, when the stent is deployed, its two ends press firmly against thebody vessel thereby creating entrance regions for flow through the stentwherein the stent is essentially flush with the body vessel, so thatdrag and turbulence are minimized. It is known that turbulence,especially in blood vessels in and near the heart, is directly linked tothrombus formation.

In certain applications it may be desirable for the stent to featureuneven contour to help anchor it in place. An example is shown in FIG.21, where depressions are placed along the coil to increase the frictionbetween the coil and the tissue. Furthermore, in some cases it may beadvantageous to have a stent which is made up of two sections which areconnected to each other by a substantially straight portion of wire,preferably connecting points on the opposing loops of the two sectionswhich are not corresponding points, so that the wire does not undulypress against the wall of the body vessel where there is a naturalconstriction in the body vessel between the two sections of the stent.Preferably the connecting wire is disposed near the periphery of thestent, as shown in the end cross-sectional view of FIG. 22, to minimizethe obstruction to flow of fluids through the central portions of thestent.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made. Forexample, the profile of the plastic envelope which makes up the stentcan be varied so as to better conform with the internal shape of thebody vessel wherein it is installed.

1. An arrangement for controlling a shape of a stent body comprising anelectrical circuit for establishing an electrical current flow throughthe stent for heating the stent to cause the stent to shift from amartensite phase to an austenite phase thereby changing the shape of thestent body, the electrical circuit further being adapted to monitor aphase change of the stent from the martensite phase to the austenitephase and to control the flow of electrical current through the stent asa function of the phase change of the stent from the martensite phase tothe austenite phase.
 2. The arrangement of claim 1, wherein theelectrical circuit includes a device for sensing a change in voltageacross the stent to indicate the phase change.
 3. The arrangement ofclaim 2, wherein the electrical circuit further includes a device forcutting off the supply of electrical current to the stent immediatelyupon sensing the change in voltage.
 4. The arrangement of claim 2,wherein the electrical circuit further includes a device for cutting offthe supply of electrical current to the stent a predetermined timeinterval after sensing the change in voltage.
 5. The arrangement ofclaim 1, wherein the electrical circuit includes a device for sensing achange in current through the stent to indicate the phase change.
 6. Thearrangement of claim 5, wherein the electrical circuit further includesa device for cutting off the supply of electrical current to the stentimmediately upon sensing the change in current.
 7. The arrangement ofclaim 5, wherein the electrical circuit further includes a device forcutting off the supply of electrical current to the stent apredetermined time interval after sensing the change in current.
 8. Thearrangement of claim 1, wherein the electrical circuit comprises atleast one resistance therein selected as a function of a resistivity ofthe stent.
 9. The arrangement of claim 1, wherein the electrical circuitcomprises at least one resistance therein selected as a function of athermal conductance of the stent.
 10. The arrangement of claim 1,wherein the electrical circuit is adapted to monitor the phase change ofthe stent in accordance with an abrupt change of at least one of (a) aresistivity of the stent and (b) a thermal conductivity of the stent.11. The arrangement of claim 1, wherein the stent is formed of a shapememory material.
 12. The arrangement of claim 1, wherein the stent isformed of nitinol.
 13. An arrangement for controlling a shape of a stentbody, comprising: an electrical circuit adapted to establish anelectrical current flow through the stent to heating the stent to causethe stent to shift from a martensite phase to an austenite phase tothereby change the shape of the stent body, the electrical circuitfurther adapted to monitor a phase change of the stent from themartensite phase to the austenite phase and to control the flow ofelectrical current through the stent as a function of the phase changeof the stent from the martensite phase to the austenite phase.
 14. Asystem, comprising: a stent; and an arrangement configured to control ashape of a stent body of the stent, the arrangement including anelectrical circuit adapted to establish an electrical current flowthrough the stent to heating the stent to cause the stent to shift froman martensite phase to an austenite phase to thereby change the shape ofthe stent body, the electrical circuit further adapted to monitor aphase change of the stent and to control the flow of electrical currentthrough the stent as a function of the monitor of the phase change ofthe stent.
 15. The system of claim 14, wherein the electrical circuitincludes a device adapted to sense a change in voltage across the stentto indicate the phase change.
 16. The system of claim 15, wherein theelectrical circuit includes a device configured to cut off the supply ofelectrical current to the stent immediately upon sense of the change involtage.
 17. The system of claim 15, wherein the electrical circuitincludes a device adapted to cut off the supply of electrical current tothe stent a predetermined time interval after a sense of the change involtage.
 18. The system of claim 14, wherein the electrical circuitincludes a device adapted to sense a change in current through the stentto indicate the phase change.
 19. The system of claim 18, wherein theelectrical circuit includes a device configured to cut off the supply ofelectrical current to the stent immediately upon a sense of the changein current.
 20. The system of claim 18, wherein the electrical circuitincludes a device adapted to cut off the supply of electrical current tothe stent a predetermined time interval after a sense of the change incurrent.
 21. The system of claim 14, wherein the electrical circuitincludes at least one resistance selected as a function of a resistivityof the stent.
 22. The system of claim 14, wherein the electrical circuitincludes at least one resistance selected as a function of a thermalconductance of the stent.
 23. The system of claim 14, wherein theelectrical circuit is adapted to monitor the phase change of the stentin accordance with an abrupt change of at least one of (a) a resistivityof the stent and (b) a thermal conductivity of the stent.
 24. The systemof claim 14, wherein the stent is formed of a shape memory material. 25.The system of claim 14, wherein the stent is formed of nitinol.